The invention concerns a hydraulic assembly for a cylinder head of an internal combustion engine comprising a hydraulically variable gas exchange valve train comprising:                a hydraulic housing comprising at least one driving side master unit, at least one driven side slave unit and at least one actuable hydraulic valve,        at least one medium pressure chamber extending in the hydraulic housing,        at least one high pressure chamber extending in the hydraulic housing and arranged in transmitting direction between the associated master unit and the associated slave unit while being able to be connected through the associated hydraulic valve to the associated medium pressure chamber,        at least one low pressure chamber extending in the hydraulic housing and serving as a hydraulic medium reservoir while being connected through a throttling point to the associated medium pressure chamber,        and a valve body which is displacably received in direction of a hydraulic medium flow between the medium pressure chamber and the low pressure chamber in the hydraulic housing and serves to form the throttling point, said throttling point comprising two flow cross-sections of different sizes for the hydraulic medium flow as a function of the position of the valve body.        
A hydraulic valve of the pre-cited type is disclosed in the not pre-published document DE 10 2009 011 983 A1. In the hydraulic assembly proposed in this document, all the important components required for the hydraulically variable transmission from cam lobes to the gas exchange valves as well as the pressure chambers are arranged in a common housing. The throttling point which connects the medium pressure chamber to the low pressure chamber which serves as a hydraulic medium reservoir is configured such that the hydraulic medium flowing from the medium pressure chamber into the low pressure chamber must pass through a throttling cross-section, and a low-throttling flow cross-section is provided for the hydraulic medium flow in a reverse direction from the low pressure chamber into the medium pressure chamber. The low throttling in this direction of flow is meant to provide a sufficiently fast availability of a hydraulic medium reservoir for the medium pressure chamber during a cold start of the internal combustion engine.
However, tests carried out by the applicant have shown that a thus configured throttling point promotes the leakage out of the high pressure chamber and the medium pressure chamber and that, already within a few days of standstill of the internal combustion engine, the leakage-compensating low pressure chamber can get emptied. As a consequence, this low pressure chamber is no longer available as a hydraulic medium reservoir during the cold start of the internal combustion engine, and the air volume collected in the meantime in the medium and/or high pressure chamber impedes or prevents, due to its high compressibility, an opening actuation of the gas exchange valves which would be adequate for the starting operation.
These problems are true in a comparable manner for throttling points with constant throttling cross-sections as disclosed in DE 10 2007 054 376 A1.